This invention relates to release surfaces utile for release liners and adhesive contacting release surfaces for self-wound tapes among other applications.
A major utility of the invention concerns an improved release liner (or backing) for use in combination with a pressure-sensitive adhesive layer and a face stock preferably for label applications. In such combinations, the release liner protects the pressure-sensitive adhesive (PSA) prior to the label being used and is removed immediately prior to application of the label to another surface.
Additionally, the release liner serves to facilitate cost effective manufacture of rolls or sheets of labels. It also functions as a carrier of labels for dispensing in automatic labeling operations and for computer printing in EDP applications. The performance attributes of a release liner are critical to both the manufacture and end-use application of pressure sensitive adhesive labels.
In conventional practice, the release liner is provided as a silicone layer on a support layer having high holdout, i.e., the support layer on which the silicone layer is deposited is resistant to silicone penetration. Where the support layer is paper, a special and, therefore, expensive paper, such as a super-calendered or densified glossy paper, is required. One currently accepted way of applying a silicone release composition to a high-holdout support layer is by solvent coating. Growing concern over the environment has imposed stringent restrictions regarding recovery of the solvent used in applying the solvent based silicone to the high-holdout backing paper or other materials.
An alternative to this is to use 100% solids silicone release compositions. These are supplied with a viscosity (usually &lt;2000 cps) suitable for roll-coating techniques. Application of these to porous substrates such as low cost papers, machine finished (MF) or machine glazed (MG) papers, finds these materials to soak into the paper (penetrate the paper surface) to give ineffective coverage of the paper fibers unless excessively high quantities of expensive silicone are used. Ineffective coverage of the paper fibers provides unsuitable release liners for PSA applications especially where high speed convertibility is an essential performance feature.
A major application for a release liner is as part of bulk rolls of laminate consisting of the release liner, a face stock between which there is contained an inherently tacky self-adhesive, or pressure-sensitive adhesive. The adhesive may be permanent or repositionable. The rolls are converted by printing label information on the face stock, die cutting the labels through the face stock and adhesive to the surface of the release liner, followed by removal of the matrix surrounding the labels thus leaving a plurality of labels on the release liner.
It is important that the force required for release be sufficiently low for the intended application, but not so low that the die cut labels will release or predispense from a moving web turning a corner or remove with the waste matrix during its removal. The release force should also be not so high that the matrix is broken during its removal.
It would be desirable to provide a release liner which can be manufactured in a more economical fashion under a condition which eliminates the risk of environmental pollution, and in which the release level can be controlled at a variety of peel rates so as to enable high-speed convertibility, i.e., die cutting, matrix stripping.
Some attempts have been made to formulate release liners under environmentally safe conditions. U.S. Pat. Nos. 4,618,657 and 4,713,410 to Katchko, et al., propose the application of a reactive silicone as a component of an aqueous emulsion which also contains a blend of a fluid hydroxyl-functional resin, which is a fatty acid, or fatty alcohol-containing polyester, and a crosslinking agent for the hydroxy resin. The silicone portion is either a hydroxy- or alkoxy-functional polysiloxane, or a vinyl-addition type silicone.
The hydroxyl-functional polysiloxanes are reactive with the hydroxyl-functionality in the polyol-modified polyester resin. Alkoxy-functionality is also reactive with hydroxyl-functionality in the polyester resin, and a tin-based catalyst assists these condensation reactions. The crosslinker (hydrogen-functional polysiloxane) of the vinyl-addition type silicone can also react with hydroxyl functionality of the polyester resin or with unsaturation in the polyester resin, as well as the vinyl portion of the vinyl-silicone base polymer.
The polyester resins described in the '657 patent are said to be of low molecular weight and are further defined in the '410 patent to be free flowing liquids at room temperature. As such, they too can permeate into porous surfaces such as low cost, MF or MG papers in a manner similar to the 100% solids silicones. Their primary utility lies in the use of less-expensive polyester resins with silicone emulsions to reduce the overall cost of the release composition.
Among other difficulties with the systems described in the '410 and the '657 patents is that stratification must also occur, with the polyester resin stratifying to the paper and the silicone stratifying or "blooming" to the surface of the resin to form, in essence, a paper-resin-silicone laminate. Stratification takes time, and this results in liners having performance properties which are dependent upon processing speed.
The technology of the '657 and '410 patents has been evaluated, and the systems provided therein gave backings exhibiting excessive resin penetration into the machine-glazed paper backing and an appearance which would preclude acceptance in the marketplace. After two weeks at ambient temperature, these constructions developed a strong, offensive odor. Release values, when used in combination with a commercial emulsion acrylic adhesive, were too high (i.e., 50 to 55 N/m) at room temperature for all label applications. When the silicone constituent was varied, release could be lowered to 25 to 30 N/m at room temperature, but a greater amount of silicone was required. Appearance remained poor, with heavy streaking. Keil aging values were not obtained.
U.S. Pat. No. 4,362,833 to Mune, et al, describes a system where a condensation-curable, hydroxyl-functional emulsion silicone system is used in combination with aqueous resins having film-forming ability and bearing hydroxyl- and/or carboxyl-functional groups. The silicone content is high, 50-80%. In addition to having limited utility in respect to emulsion acrylic adhesives, condensation-curable silicones cure at an extremely slow rate, making them commercially unattractive for high-speed web processes.
Similarly, Japanese Patent Publication Sho 51-139835 to Vemura describes an emulsion, contained therein in a reactive condensation curable silicone, where the catalyst is an acid, alkali, amine, or organic metal salt (usually tin salt) used in combination with a resin emulsion. Where the resin may be inert, it is preferably reactive with the silicone. This technology would also be unsuitable for high-speed web processes.
The present invention relates to the improvement in the release surfaces utilizing aqueous-based silicone resin systems not heretofore contemplated in the art and adaptive to a broad-based utility in all phases where release surface is required, including all applications where controlled release is required.